SiC single crystals are thermally and chemically very stable, superior in mechanical strength, and resistant to radiation, and also have superior physical properties, such as high breakdown voltage and high thermal conductivity, compared to Si single crystals. They are therefore able to exhibit high output, high frequency, voltage resistance and environmental resistance that cannot be realized with existing semiconductor materials, such as Si single crystals and GaAs single crystals, and are being considered ever more promising as next-generation semiconductor materials for a wide range of applications including power device materials that allow high power control and energy saving, device materials for high-speed large volume information communication, high-temperature device materials for vehicles, radiation-resistant device materials and the like. In particular, there is a demand for p-type SiC single crystals with low resistivity, in order to obtain ultra-high voltage-resistant elements that are considered promising for applications in electrical power systems and the like.
Typical growth processes for growing SiC single crystals that are known in the prior art include gas phase processes, the Acheson process, and solution processes. In gas phase processes, for example, sublimation processes have drawback in that grown single crystals have been prone to hollow penetrating defects known as “micropipe defects”, lattice defects, such as stacking faults, and generation of polymorphic crystals. However, most SiC bulk single crystals are conventionally produced by sublimation processes. In the Acheson process, heating is carried out in an electric furnace using silica stone and coke as starting materials, and therefore it has not been possible to obtain single crystals with high crystallinity due to impurities in the starting materials.
Solution processes are processes in which molten Si is formed or a molten liquid comprising another metal dissolved in molten Si is formed in a graphite crucible and C is dissolved into the molten liquid, and a SiC crystal layer is deposited and grown on a seed crystal substrate set in the low temperature zone. Solution processes can be expected to reduce defects because crystal growth is carried out in a state of near thermal equilibrium, compared to gas phase processes.
For growth of a SiC single crystal by a solution process, there has been proposed a method in which Al is added to a Si—C solution in order to grow a low-resistance p-type SiC single crystal (PTL 1).